1. FIELD OF THE INVENTION
This invention relates to the field of stabilization of MBS impact modifiers and PVC compositions containing such impact modifiers.
2. DESCRIPTION OF PRIOR ART
MBS modifiers have poor resistance to thermal oxidative degradation. This poor resistance may manifest itself most strongly during such procedures as isolation and oven drying. The least severe manifestation is slight discoloration, while dryer fires are an extreme, though not uncommon, consequence of failure to stabilize MBS modifiers.
Antioxidants are therefore incorporated into the MBS modifiers to reduce the risk of discoloration and fire during isolation and drying. The phenolic antioxidants exemplified by ##STR2## wherein R.sub.10 and R.sub.12 each represent hydrogen or a straight or branched C.sub.1 to C.sub.4 alkyl group, and R.sub.11 represents hydrogen or a C.sub.1 to C.sub.4 straight or branched alkyl group or --CH.sub.2 CH.sub.2 CO.sub.2 R.sub.13 where R.sub.13 is a C.sub.8 to C.sub.20 straight or branched alkyl group, have been employed, particularly in concert with thiodipropionate synergists, to stabilize MBS modifiers. These systems are adequate in many respects, but are hampered by their tendency to develop colored impurities as a by-product of their antioxidant function. This production of colored impurities is particularly evident in the commercially important spray drying processes that utilize partially inerted atmospheres.
Belgain Patent 853,844 and Japanese Patent 142537 are illustrative.